The Labour leader claimed that backing the murderous jihadist cult based in Iraq and Syria was simply "expressing a political point of view".
He also raised questions about attempts to stop Islamist fighters returning to the UK from Syria.
Mr Corbyn's remarks, made in a House of Commons debate, were highlighted following the London Bridge terrorist outrage, which Islamic State has claimed responsibility for.
Jeremy Corbyn once insisted that ISIS backers should not be prosecuted for their views
Mr Corbyn said, according to records of parliamentary proceedings: “I have no support for ISIS whatsoever, and obviously that should apply to someone who has committed crimes, but we should bear in mind that expressing a political point of view is not in itself an offence.
"The commission of a criminal act is clearly a different matter, but expressing a point of view, even an unpalatable one, is sometimes quite important in a democracy.
"We should be slightly cautious about announcing that we will start to deal with people on the basis of a general view that they have expressed."
He also raised questions about attempts to stop Islamist fighters returning to the UK from Syria
Mr Corbyn, then a backbench Labour MP, said he had "no truck with those who commit those barbaric acts".
Expressing a political point of view is not in itself an offence
But he went on to urge Theresa May, who was the home secretary at the time, against creating further "legal obstacles" to prevent jihadists returning to the UK after fighting with extremist groups in Syria.
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He called on the Government to avoid the knee-jerk reaction of saying: “These are bad fighters and those are good fighters, so we will ban these and allow those in."
Mr Corbyn's remarks were highlighted following the London Bridge terrorist outrage
During the debate, he added: "We are living with the consequences of the war on terror of 2001, and if we continue to try to create legal obstacles and make value judgments about people without considering the overall policy we are following, we will return to legislation such as this again and again, year after year."
Mr Corbyn's remarks emerged as the Tories stepped up their accusations that Labour is weak in security.
Mr Corbyn also faced criticism from the Tories after reaffirmed his opposition to the police's "shoot-to-kill" policy in the face of terrorist threats.
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In the past, the Labour leader has condemned the policy as "quite dangerous and often counterproductive".
And he refused to change his position in the light of the terrorist outrage at London Bridge, where armed police gunned down three knife-wielding fanatics wearing fake suicide jackets.
He said: "I have not changed my mind on shoot-to-kill."
Mr Corbyn also faced criticism over his opposition to the police's 'shoot to kill' policy
Tory Security Minister Ben Wallace said: “Jeremy Corbyn cannot hide from his record of consistently opposing the powers the police and security services need to keep the United Kingdom safe.
"The police should not have to second-guess whether they should pull the trigger against a threat to the public.
“For thirty years Corbyn has put his support for Britain’s opponents ahead of national security, yet four days before polling day he cynically wants to fool voters into thinking he will keep Britain safe.
“Jeremy Corbyn and his top team have shown an abdication of leadership."